Anzalone said that Lewis was terminated as part of a series of job cuts Arrowhead announced on Tuesday. The company is headquartered in Pasadena, CA, but houses its research and development operations in Madison, WI, where Lewis is based. Anzalone said that 25 of the 34 employees who were laid off worked in Madison. Following the cuts, he said, Arrowhead has a total of 84 employees, including 66 in Madison.

At the time Arrowhead announced the layoffs, it also said it had decided to cancel the development of three experimental drug candidates for liver disease. The news triggered a mass sell-off of Arrowhead stock. Shares in the company closed the trading day at $1.44 each on Wednesday, down 67 percent from the previous day’s closing price of $4.39 a share. (Arrowhead made the announcements minutes after the closing bell rang on Tuesday.) Shares gave off another 8 percent on Thursday.

Arrowhead has not yet determined whether it will give the title of chief scientific officer to another employee, Anzalone said. He added that chief operating officer Bruce Given will assume some chief scientific officer duties after Lewis’s last day at Arrowhead, which is Dec. 13, according to the filing.

Anzalone called Lewis a “fantastic scientist” and a “pioneer in RNAi.” That’s short for ribonucleic acid interference, a method of attacking viruses that has been part of the design of several Arrowhead drug candidates. This process, which is also known as “gene silencing,” involves destroying messenger RNA from viruses before the infected cell turns those RNA into proteins that spread the infection.